[Hormone-producing thyroid carcinoma in a female 10-year-old Australian Cattle Dog].

A 10-year-old female Australian Cattle Dog was referred for investigation of a mediastinal mass. During clinical examination, a mandarin- sized mass was detected ventrally of the trachea. According to computed tomographic (CT) findings and cytology of a fine-needle aspirate, thyroid neoplasia was suspected. The assumed tumour affected the left thyroid gland and was staged as stadium II because of enlargement of the ipsilateral cervical and retropharyngeal lymph nodes. The left thyroid, including the parathyroid gland, was removed surgically with a guarded prognosis. Histological examination revealed a follicular thyroid adenocarcinoma. The total thyroxine level was slightly elevated preoperatively. Postoperatively, the dog developed persistent hypothyroidism. A mediastinal mass could not be identified on the CT scan.